All woods have a fibro-vascular tissue composed of cellulose and its components belonging to the subdivision called spermatophytes (IV) in the plant kingdom (with the single exception of tree ferns). The spermatophytes can be further subdivided into two classifications; gymnosperms or "softwoods" and the angiosperms or "hardwoods". It must be emphasized that the terms softwood and hardwood have no bearing on the density or degree of hardness of such woods but refers to their classification. Some woods that are classified as softwoods, such as yellow pine, are physically harder than some woods that are classified as hardwoods such as aspen or basswood. Further, agniosperms can be again divided into very distinct classes; the monocotyledons or the palms, bamboos, canes and grasses and the dicotyledons (the majority of angiosperms that provides us with useful woods).
Since a living tree contains very large amounts of water, lumbermen often refer at various stages from the initial cutting of a tree up through the sawing and drying of lumber to the moisture content ("MC") of the wood. The moisture content of the wood, usually expressed in a percentage, is a ratio of the amount of water in a piece of wood that is compared to the weight of such wood when all of the moisture has been removed. One of the methods that is employed (the "moisture content on the oven-dry basis") to determine the MC of wood at any stage during the lumber production process is to weigh a given sample of wood and record such weight (the "wet weight"). The sample is then placed into an oven and heated at temperatures not to exceed 217 F until all of the moisture has been removed (the "oven dry weight") and that weight is recorded. It can be determined that the oven-dry weight has been reached when, after weighing at various intervals, the sample stops losing weight. The oven-dry weight is then subtracted from the wet weight and the resultant is then divided by the oven-dry weight. That resultant figure is then multiplied by 100 to determine the percentage of MC. The formula is represented as follows: ##EQU1## The type of units employed for the above calculation, i.e. ounces, grams, pounds, kilograms, etc., is not important as long as all weights are recorded in the same type of units since the calculations are based upon a ratio of such weights. Other methods of determining MC have been developed as well as electronic machines that compute the MC based upon known electrical and other reactions. Regardless of the method employed to determine such MC, a working knowledge of moisture content and how it affects wood is important to the present process.
When a tree such as red or white oak, fir, maple, spruce, ash or any one of the many species of trees that yield wood that is useful in the production of wood products is initially cut down, it has a MC of anywhere from about 60% to 100% (this moisture content has been found to be even higher, as much as about 200% for some species). This is called the "green moisture content" ("GMC"). Opposed to popular belief, the green moisture content does not vary greatly with the season that a log is cut. This moisture or water has to be removed or dried from the wood in order to make the wood stable and thus usable in any phases of the lumber industry that require either air dried and/or kiln dried lumber. The drying or curing of green wood thus comprises the controlled removal of water from the wood to a level where the wood becomes sufficiently stable for fabrication into various products. The "curing" process or "curing" as used herein refers to moisture removal by the controlled act of air drying, kiln drying, or a combination of both.
After a tree is felled and is sawn into lumber of various sizes and types, it is stacked in a particular manner in preparation for the drying and/or pre-drying process. During this curing process, many problems may occur that can either damage, destroy or degrade the quality of the wood and render it less desirable and in some cases, not usable at all. The sawn lumber can develop cracks in the ends ("end checks"), cracks in the internal portions of the lumber ("honeycomb" or "honeycombing"), cracks in the surface ("surface checking"), as well as many types of warps and bends ("cup", "bow", "crook", etc.). Such problems are all related to the presence of moisture in the wood itself and the movement of, and subsequent removal of, such moisture from the time a tree is felled until the completion of the curing process. The significance of the removal of moisture during the curing process[s] becomes more understandable through a thorough understanding of the actual structure of wood itself.
The layers in a typical tree are: a) the outer bark; b) the inner bark; c) the cambium layer; d) the sapwood and e) the heartwood. The outer bark is a rough textured layer composed of dry, dead tissue that provides the tree with its first line of defense against external injury and insect infestation. The outer bark is separated form the next layer called the inner bark by a thin layer called the bark cambium. The inner bark is a soft, moist layer that contains living cells that play a role in the transfer of food to the growing parts of the tree. The cambium layer is a very small microscopic layer that is just inside the inner bark. The main function of the cambium layer is to produce both bark and wood cells.
The sapwood is composed of light colored wood and is made up of both living and dead tissues. The heartwood is the central section of the tree that is laden with resins and tannins and is basically inactive. Heartwood is formed by the transformation of sapwood as the tree ages. Both the sapwood and the heartwood are composed of many layers or "rings". These are called annual rings and each one represents the amount of growth a tree undergoes for a given year of its life. The heartwood is less permeable than that of sapwood and subsequently needs more drying time and is subject to more drying defects than sapwood. The infiltration of resins, gums and other materials in the heartwood make it more resistant to moisture flow and also make such heartwood darker in color.
The internal structure of wood is basically oriented around the flow of moisture since a tree distributes the nutrients it requires for growth in a liquid medium or sap. A basic element of such internal structure is the wood cell. There are two basic distribution processes that sap movement can occur in a tree. Such processes are called diffusion and conduction. In a wood cell, diffusion occurs when sap passes through the cell walls by the action of the protoplasm which covers cells that are rather new or young. Conduction occurs when the cells age somewhat and lose their protoplasm and develop log pits or spot through which, sap passes easily. As some cells age, they might also break down at the ends and form tracheal vessels, sometimes referred to as the "through passageways", which utilize conduction as a transfer medium. The basic unit of a tree or the wood cell is characterized by different elements that utilized either one or both of such distribution methods. Each wood cell has a cell wall structure composed of several different layers and a central cavity. The cell wall is composed of lignin, cellulose and hemi-celluloses. These wood cells which are tube-like in shape have different functions dependent upon their particular anatomical construction. The tracheal vessels are longitudinal tubes composed of dead material. They are relatively long and large in diameter and play a role in the upward conduction of sap. The tracheids, closely related to the tracheal vessels, are somewhat narrower and shorter and also play a role in the upward conduction of sap. Tracheids provide a function as mechanical tissue, especially in woods that lack wood fibers, such as coniferous woods. Wood fibers are longitudinal strands of thick walled cells (long and pointed) which are lignified and are usually of dead material. Parenchyma cells are present in wood and medullary rays and are therefore longitudinal and radial. Parenchyma cells move sap by both conduction and diffusion and work as the food digestion and storage of organic materials, including oil, sugar, starch, etc. The only place in wood where air spaces occur between the cells is between parenchyma cells.
There are other types of anatomical elements that are important, and affect the flow of moisture within the wood. One such element is referred to as a pit and exists in several forms. Pits are small, valve-like openings that connect wood cells thereby becoming an important means of water transfer. Tracheids develop what is referred to as a "bordered pit" or a thin spot through which sap can pass more easily from cell to cell. As the walls of some tracheids become lignified, there is an increase of permeability to water as cell walls containing lignin allow a more free passage of water and do not swell as much as non-lignified cell walls. The pits however can become encrusted with certain substances that obstruct the flow of water and become in effect, clogged. Additionally, a characteristic referred to as "aspiration" can occur in some woods during the curing process to cause a restriction to the flow of water thereby to extend the curing period.
Another anatomical element, the tyloses, play a role in the movement of sap throughout the body of the tree and therefore affect the curing process. Tyloses are sac-like portions of the parenchyma cells that have pushed through the pits and moved into the cavities of the tracheids and tracheal vessels. Sometimes they become so numerous in certain species of wood that they obstruct the circulation of sap and can totally block up such movement of sap except in the outer portions of the sapwood. Since tyloses do not have a distinct nucleus of their own, they are not different cells but are an outgrowth from a medullary ray or parenchyma cells that expands into an empty cavity of a tracheal vessel. Since tyloses restrict circulation of sap within the wood, then woods that are high in tylose content, i.e. white oak, will have reduces permeability. In contrast, a wood that is lower in tylose content such as red oak, which usually has no tyloses in its tracheae, is more permeable.
Other anatomical features of a tree that are related to the movement of moisture within the wood are the medullary rays or "pith" rays which radiate out from the pith or central core of the tree stem. Unlike other cells in the tree, the medullary rays are perpendicularly aligned with the tree stem instead of longitudinally as are most other type of wood cells. In some types of trees, the medullary rays are quite conspicuous such as oak, beech and sycamore and compared to others, such as pines and conifers, such rays are microscopic. The medullary rays consist largely of parenchyma cells that are used in the conduction of food and nutrients to the cambium layer where such elements are used in the formation of new tissue. During the curing process for wood, water flow is usually faster around medullary rays than in surrounding cells making this part of the wood dry faster. Additionally, medullary ray cells are typically weaker. Species such as oak and beech which have large, pronounced rays have traditionally needed to have special care during the curing process to prevent checks, honeycombing and splits around such ray cells.
The dissection and nomenclature of wood therefore, plays a major role in the curing process since the anatomical structure of different species of wood all seem to be related to the restriction or movement of moisture in some manner or form. From the moment that a tree is felled, some form of moisture loss begins to take place from the sawn ends, the cuts to remove the limbs, abrasions that removed the bark, etc. All woods lose or possibly gain moisture in an attempt to reach a state of equilibrium with the moisture present in the surrounding air. As wood loses moisture, it begins to shrink and develop internal stresses which are relieved by the formation of cracks. Because moisture moves much faster from the cut ends of the wood than from the side or edge grains, then end checks or splits will occur within a very short time if a substantial moisture loss occurs from such ends. Usually, if the tree is sawn into lumber within a relatively short period after being felled, such as one week, such incidental moisture loss is not significant. However, if ambient conditions are very hot and dry, long holding periods for logs have to be accompanied by watering the logs to retard moisture loss or by waxing or coating the cut ends, limb cuts and other abrasions. Once the protective bark is removed and the log is cut into lumber, the moisture migration begins. Such moisture migration from lumber must be controlled and restricted in order to prevent drying defects.
Under conventional practices, as a given log is sawn into lumber, the individual boards of uniform thickness are stacked with spacing between them with precisely sized and positioned spacer boards or "stickers" usually about 3/4".times.3/4".times.48" long between the layers (a process known as "stickering" in the industry). Stickering promotes an even amount of exposure to the atmosphere (either natural or created) within the bundle or stack that has been created. The ends of each board are then end coated with a special form of wax, or such other suitable coating, to retard end checking because of the accelerated movement of moisture from the end grain of all woods (as compared to moisture movement from a side or edge grain). The bundle is normally pre-dried or air dried by placing the bundle in an area of controlled exposure to air, heat, and moisture to permit a controlled escape of moisture necessary for the "pre-drying" or "air-drying" phase. The pre-drying phase is effective to remove some or all of the "free" water that is present in the cells of the wood itself. In some instances, however, the pre-drying phase may be omitted. As used in the specification and claims herein, "free" water is defined as that moisture contained within the cell cavities of the wood. Because such free water is held less tightly than the remaining moisture or water in the wood, less heat energy is required to remove such free water during the subsequent kiln drying process applied after the pre-drying or air-drying phase. This is in contrast to "bound" water which is defined as that water that is contained within the cell walls themselves and requires higher application of energy to affect moisture reduction to a predetermined level. Most of the drying defects and problems associated with kiln dried lumber occur during the removal of the bound water.
The removal of free water brings the subject wood to a critical level in kiln drying known as the "fiber saturation point". As used herein, the term "fiber saturation point" is defined as the point where the cell walls are still saturated and all of the free water has been removed from the cell cavities. For most purposes the fiber saturation point is about 30% although it may be different for some species (possibly lower). Since wood dries from the outside to the inside (primarily by diffusion and/or capillary action), there is usually a differential between the MC of the surface of a board and the interior MC during the curing process. This differential, called a "gradient" between the inside MC and the outside MC, is usually between 15% to about 45%. Even though the average MC might be 30%, many of the interior cells might not be at the fiber saturation point. Since it has been established that the removal of the bound water causes many of the problems associated with the curing process, it is important to determine when the fiber saturation point is reached.
The "equilibrium moisture content" ("EMC") is another important factor that is conventionally used in the curing of woods. As used herein, the equilibrium moisture content is defined as that point at which the MC of a given board reached a balance with the outside temperature and relative humidity (the surrounding atmosphere of such board or the "RH"). There are other factors that could have a small effect on the EMC, such as the wood species or previous moisture content, for example. Conventional kiln drying includes a continuous manipulation of temperature and relative humidity to keep the progression of the change in EMC at a pre-determined rate of reduction. During the curing period, the relative humidity is constantly monitored. The relative humidity can be determined and monitored by several different methods employing different types of equipment. A common method to determine relative humidity is by the use of a wet-bulb thermometer simultaneously with a dry-bulb thermometer. A wet-bulb thermometer is a standard thermometer that has the sensor portion covered by a muslin wick that is kept wet with water. A dry-bulb thermometer conversely is the same temperature sensing device less the wet muslin wick. By monitoring the difference in temperature between the wet-bulb and dry-bulb thermometers (the "wet-bulb depression") and knowing the dry-bulb temperature, a chart can be consulted to determine the relative humidity of the air. Although other methods of determining the RH are effective, the wet bulb/dry bulb method is used with this invention.
The terms including their definitions as set forth above for the curing process are utilized in the conventional curing of wood and are important in understanding the forces that move moisture within a given piece of wood. These forces, primarily by diffusion and capillary action, when not controlled, cause most of the drying defects: i.e. cracks, surface checks, end checks, cups, bows, bends and other types of warps; honeycombs and honeycombing. Conventional curing techniques require complicated controls to inhibit the movement of moisture to prevent such defects from happening. As indicated above, wood dries from the outside in, therefore uncontrolled or rapid drying can cause a situation where the outside of a board dries too rapidly and is permanently "set" causing a situation known as "case hardening". As drying continues, the interior of the board develops core stresses that are unable to contract, thereby developing interior cracks (honeycombs or honeycombing). Because of this effect, the thickness of a given board being cured is of particular importance to such curing processes.
In the drying of wood, particularly a relatively thick lumber item, the rate of drying from the surface region is faster than from the interior. Thus, the surface regions are dried to the fiber saturation point at which shrinkage begins before the inwardly adjacent regions begin to shrink. The surface tries to shrink but the shrinkage is opposed by the non-shrinking adjacent regions. A stress is set up which may result in structural defects, such as checking, cupping, twisting, or warping. Also, if the surface regions become quite dry, both heat and mass transfer are reduced. It is thus necessary to maintain the surface regions as moist as possible relative to the rest of the wood to reduce degrading and defects. Normally this is accomplished by controlling the humidity of the circulating air so that equilibrium between the vapor pressure of air and that of the wood maintains a high moisture content of the wood. However, high equilibrium moisture contents are established only under conditions of high relative humidity which may be difficult to obtain.
The drying of woods, especially when the variety of species are considered, is a very specialized and exacting process. Very complex pre-drying and kiln drying schedules, most of which are effective only for a given locality and climate, have been normal heretofore for the wood drying industry.
Heretofore, and particularly for hardwoods, a pre-drying phase is often utilized for reducing the MC in the wood to an acceptable level prior to kiln drying normally by the slow removal of the MC over several days or more. It has been accepted heretofore that the MC of hardwood should not be reduced more than about 21/2% a day for oak and similar species in order to minimize any drying defects or problems that may develop from the kiln drying process where high heat is utilized. An average of about 13/4% reduction in MC for oak and similar species of hardwood in a 24 hour period has been normal heretofore. The pre-drying phase is normally effective for reducing the MC at least 20% and may be over a period of several days or several weeks. A common pre-drying phase comprises placing the cut lumber which has been stickered in open air for a period of several days or weeks before the kiln drying. Generally, the pre-drying phase does not utilize any artificial or generated heat but utilizes ambient condition or heat for effecting the pre-drying phase. Green wood has a MC of at least about 60% when the tree is felled and the loss of moisture by air-drying and other processing is effective to reduce the moisture content at least about 20% prior to kiln drying.
Heretofore, starting from the felling of a tree, it has been common to reduce the moisture content of the green wood as quickly as possible. No attempt has been made heretofore to maintain the moisture content (MC) of the green wood as close as possible to the original MC of the wet log. Accepted practices have restructured the amount of MC that could be removed from the green wood over a twenty-four (24) hour period to about 21/2% for oak and similar species of hardwood so that drying defects and other problems that develop from the kiln drying process do not occur. An average MC removal for hardwood of about one to 11/2% is normal for a Southern climate. For commercial usage, the moisture content for hardwood that is made into furniture or similar wood products is reduced to a final MC of between 6% to 10%. The moisture content of softwoods, such as those used in the construction industry for homes and buildings is required to be reduced to a final MC between 15% and 20%. Thus, drying times for kiln drying, particularly for hardwoods, normally have been several days. As most drying procedures heretofore do not attempt to retain the MC of the log after felling, the MC of the lumber after pre-drying is generally less than about 35% to 50%, particularly for hardwoods. The kiln drying is then effective to reduce the MC to a total MC of between 6% and 10% for most hardwoods, and a total MC of between 15% and 20% for most softwoods.
Many softwoods, such as southern yellow pine, as well as some hardwoods such as appalachian oaks, for example, do not undergo a pre-drying phase and often are placed directly in a dry kiln within a few days after cutting from the forest. In this event, the original MC in the pine wood has not been reduced over about 10% to 15%. Yet the time for curing the pine softwood in a dry kiln is about two (2) to three (3) days by heating the wood to about 180 F to 210 F and maintaining the heat at this level throughout the drying schedule.
The preventing of stain in wood, particularly hardwood is desirable since hardwood is usually utilized for furniture. Sawn lumber develops several types of stains which occur during the drying process. Most stains occur between the time that a tree is felled and during the drying process. Stains form a substantial problem, particularly for hardwoods which are utilized for furniture.
Such stains fall into two very troublesome classes of stains, sap stain or blue stain caused by a fungus and chemical stains caused by the action of enzymes that are contained in the wood. Blue stain is a fungal stain that occurs in the sapwood of the tree. The sapwood comprises the living layers (parenchyma cells), growing layers (cambium layer) and semi dormant cells which take part in the life processes of the tree that surround the heartwood. The heartwood contains stabilized cells that are hardened and laden with tannin, natural chemicals and resins. The stability of the cells in the heartwood and the presence of tannin, as well as the lack of the sugars and starches, prevent the intrusion of the discolorations due to the blue stain and the chemical stains in such heartwood cells.
Blue stain is caused by fungal activity which is promoted by four main elements. Those elements are: a) temperature above 50 F (a reason that blue stain is more troublesome in the southern United States); b) presence of oxygen; c) presence of moisture; and d) presence of sugar and starch occurring naturally in living cells of the sapwood. The elimination of one of these elements is normally effective to control blue stain.
Chemical stains such as sticker stain, sticker shadow and interior graying also occur in the sapwood and are caused by the oxidation of enzymes that are present in the living cells of the sapwood fibers. The control of chemical stains is effected by controlling the lot exposure of oxygen to the sawn lumber and the completing of the drying cycle of the wood as quickly as possible. However, drying schedules that are presently used have not been very effective in preventing stain growth. My prior application Ser. No. 08/859,848 filed May 21, 1997 is particularly directed to the preventing or minimizing stain in green wood.
Reissue Pat. No. RE28,020 reissued May 28, 1974 discloses a kiln drying process designed to reduce the kiln residence time with minimum structure stressing. The rate of moisture removal is maintained substantially constant, or accelerated constantly, over the drying period. The temperature of the heating fluid is increased above the temperature of the wood and this condition is maintained until the moisture content of the wood is reduced to the desired level. The RE28,020 patent does not show any reduction in the temperature of the heating fluid to a temperature below the temperature of the wood during the drying process for removal of internal heat from the wood, and does not show the exposure of the wood after heating to an outside cooling fluid surrounding the wood for reducing the temperature and humidity of the wood to the temperature and humidity of the outside cooling fluid.
It is an object of this invention to provide a process for the accelerated curing or drying of green wood that substantially reduces the curing time while providing minimal drying defects, such as checking or warping.
It is a further object of this invention to provide a process for the accelerated curing or drying of green wood that is also effective in preventing or minimizing staining of the wood.